[racket-dev] [plt] Push #21701: master branch updated

On Thu, Dec 9, 2010 at 3:50 PM,  <robby at racket-lang.org> wrote:
>
> 18dacad Robby Findler <robby at racket-lang.org> 2010-12-09 10:20
> :
> | a first attempt at a rewording of the blame error messages to admit the possibility that the contract was wrong and also to claim that fixing the blamed module or the contract is all that is required
> |

For anyone following at home, the change turns this message

/Users/clklein/tmp/contract-violator.rkt:9.17:
  (file /Users/clklein/tmp/contract-violator.rkt)
 broke the contract (-> any/c any/c any/c) on #:equiv argument of
test-->; expected a procedure that accepts 2 mandatory arguments
without any keywords, given: 1

into this one

/Users/clklein/tmp/contract-violator.rkt:9.17: found a contradiction
between the contract (-> any/c any/c any/c) on #:equiv argument of
test--> for
  (file /Users/clklein/tmp/contract-violator.rkt)
 and its implementation; expected a procedure that accepts 2 mandatory
arguments without any keywords, given: 1; to fix adjust either the
contract or the implementation of
  (file /Users/clklein/tmp/contract-violator.rkt)

Personally, I don't see the phrase "m broke the contract c" as
denying, or even suggesting, that c is wrong.

But I know that contracts are specifications and that specifications
have bugs too, so I'm not the reader you're worried about. If we're
seeing people misinterpret the message, I wonder if it would be better
to tack something like "The problem may be with m or with c" onto the
end of the old message.

That said, I'm a little worried about giving so much debugging advice
in the message. Consider this program.

#lang racket/load

(module m racket
  (define non-neg-num/c
    (and/c number? (not/c negative?)))
  (define abs-codom/c
    ;non-neg-num/c
    number?)
  (define abs values)
  (provide/contract
   [abs (-> number? abs-codom/c)]
   [sqrt (-> non-neg-num/c non-neg-num/c)]))

(module n racket
  (require 'm)
  (sqrt (abs -7)))

(require 'n)

The resulting message says to fix either the contract on sqrt or the
module n, but the problem is really in the definition of abs (and its
contract).

Yes, n has a problem too (it should not use sqrt without protecting
itself with a stronger contract on abs), but someone who would misread
the original error message would probably not recognize this problem.